The present invention relates to the preparation of anhydrous sodium sulfide by heating water-containing sodium sulfide under reduced pressure.
Processes for preparing an anhydrous sodium sulfide from Na2S.9H2O, Na2S.6H2O or Na2S.5H2O are already known from the prior art.
According to EP-B-0345 136, the monohydrate is prepared in the first dehydration step. The elimination of water requires a time of 2 hours or more.
The remaining water contained in the monohydrate is then removed at a temperature of from 90 to 200xc2x0 C. and under a reduced pressure of approximately from 2.66 kPa (20 torr) to 66.5 kPa (500 torr).
A similar, two-step process for the preparation of crystalline anhydrous sodium sulfide is described in EP-B 0361 998.
The disadvantage of those known processes is that it is necessary to wait for the reaction in the first step, for example to the monohydrate, to be complete (EP-B-0345 136) before this compound can be conveyed to the second temperature step.
A product that is available commercially is a water-containing sodium sulfide having an Na2S content of especially from 60 to 62%. According to stoichiometric calculation, that corresponds approximately to Na2S.3H2O.
The introduction of hydrogen sulfide into concentrated sodium hydroxide solution yields, at temperatures above 100xc2x0 C., a sodium sulfide hydrate melt having a content of approximately from 60% to 62% Na2S.
The melt is placed on a cooling conveyor, on which it solidifies (T less than 90xc2x0 C.). Comminution generally yields a flaky product, which is then available commercially.
The object of the invention is to provide a process by means of which there is obtained a sodium sulfide that is anhydrous to the greatest possible extent.
The invention provides a process for the preparation of anhydrous sodium sulfide by heating water-containing sodium sulfide under reduced pressure, characterized in that sodium sulfide having a water content of from 35 to 45%, especially from 38 to 40%, is conveyed, with mixing, through an apparatus in which the temperature of the Na2S hydrate that is fed in (product temperature) rises continuously from approximately 20xc2x0 C. at the point at which the solid is fed in to xe2x89xa7180xc2x0 C. at the solid outlet, and a low pressure of  less than 20 torr is maintained simultaneously. In this manner, the undesired melting of the material that is to be dehydrated is prevented.
A pressure of  less than 11 torr is preferably maintained during the dehydration process.
For the process according to the invention there are used especially contact driers.
The time interval to complete the drying process of the invention is both a significant and unexpected improvement over the drying processes known in the prior art. By varying the conditions of vacuum, temperature and pressure according to the invention, the dwell time can be adjusted to complete a full cycle in less than 4 hours, and can be as low as 10 minutes. Intermediate time intervals embrace 3.5 hours, 3.0 hours, 2.5 hours, 2.0 hours, 1.5 hours, 1.0 hour or 30 minutes.
These dwell times can be completed on large and small throughput masses of sodium sulfide. A preferable throughput rate is 300 to 400 kg/h. Although this drying cycle can be accomplished at rates of 250 kg/h, 200 kg/h, 150 kg/g, 100 kg/h, 50 kg/h, 25 kg/h and lower according to the invention; it is also possible to accomplish complete drying when the mass throughput is higher than 400 kg/h by scaling up the drier apparatus to accommodate the higher mass flows. Complete and rapid drying at throughput rates can be accomplished at higher mass flows including 500 kg/h, 600 kg/h, 800 kg/h, 1000 kg/h, 1500 kg/h, 2000 kg/h, 4000 kg/h, 10,000 kg/h, 25,000 kg/h, 50,000 kg/h, 100,000 kg/h and higher.
Contact driers are to be understood as being apparatuses in which the heat from a heating surface is transferred to the solid that is in contact with this surface. Thus, the process of the invention can be understood to involve the preparation of anhydrous sodium sulfide by heating water-containing sodium sulfide having a water content of from 35 to 45% by conveying the sodium sulfide with mixing, in contact with a heated surface and transferring heat to said sodium sulfide in contact with said surface so as to raise the temperature of said sodium sulfide continuously from approximately 20xc2x0 C. at the point at which the sodium sulfide contacts the heated surface up to xe2x89xa7180xc2x0 C. where the sodium sulfide leaves the heated surface, at a low pressure of  less than 20 torr, preferably  less than 11 torr, which is maintained throughout the heating.
Such apparatuses include, for example, paddle driers and plate driers, the former generally being operated discontinuously and the latter continuously.
Plate driers that are installed vertically have proved to be especially suitable.
In these apparatuses, the plates, which are arranged from top to bottom, are combined into three or more sets of plates of different temperature zones. The solid temperature in the first set after the point at which the solid is fed in is from 20 to 70xc2x0 C., and the solid temperature (product temperature) in front of the solid outlet is greater than 180xc2x0 C.
It is a feature of the invention, that the heated surface can be in the form of a plurality of plates that are combined into sets assigned to three or more temperature zones. The heat is controlled so that the temperature of the sodium sulfide in the first set (after the point at which the solid is fed in) is from 20 to 70xc2x0 C. and the temperature of the sodium sulfide in front of the solid outlet at the end of the heating zone is greater than 180xc2x0 C.
The water-containing Na2S is preferably used in the form of scales, lamellae or flakes having an edge length of from 5 to 8 mm.
According to the invention it is possible to markedly reduce the dwell time of the sodium sulfide that is to be dehydrated in the drying process as compared with the prior art, and to make available a dehydration process which may be carried out discontinuously and, preferably, continuously.
Especially where a vacuum plate drier is used in continuous operation, the process yields an anhydrous sodium sulfide having a content of at least 98 wt. %, preferably 99 wt. %, Na2S even after dwell times in the drier of xe2x89xa790 minutes.